The origins of prosocial responses to the distress of others are sought through controlled observational studies of children in the first three years of life. Both the events in the stimulus world that promote prosocial development and the responses that constitute early prosocial acts are investigated. One purpose of the proposed studies is to determine events in the course of infants' usual experience in the home that might facilitate the acquisition of prosocial behavior, including exposure to distress-including circumstances and observation of adult models. Consequently, observations of 8-, 16-, and 24-month-old infants are conducted in the home in order to categorize the social consequences (in terms of caretakers' responses, prosocial or otherwise) of the infants' own distress. These observations serve to index the variety of responses to another's distress modeled by caretakers. An additional purpose is to describe early manifestations of prosocial responses to distress. The extent to which very young children attend to, react emotionally toward, comment upon, and attempt to alleviate the distress of their peers is examined in a daycare center and in two studies in the laboratory. The initial laboratory study examines the responses of unfamiliar peers (6-, 9-, and 12-month-old infants) to each other's fussing and crying; in the second study a play group, composed of equal numbers of 12-, 18-, and 24-month old infants, is observed over the course of 4 weeks. The laboratory studies thus permit more systematic comparison of infants of different ages and assessment of the effects of increasing familiarity on responses to each other's distress.